Abstract. -We present detailed JHK surface photometry with ellipse fits of 13 galaxies selected from previous optical observations as likely candidates for having a secondary bar or a triaxial bulge within the primary bar. We have found 7 double-barred galaxies, 3 double-barred galaxies with an additional intermediate structure with twisted isophotes, and 3 galaxies with a bar and central twisted isophotes. A global analysis of the structural parameter characteristics in the I-and K-bands is presented. Various numerical models of galaxies with bars within bars are also analysed using the ellipse fitting technique and compared to the observations. A thorough review of the possible hypotheses able to explain this phenomenon is given with emphasis on the most likely ones.

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Abstract. Surface photometry in the J and K bands of 15 southern Seyfert or starburst galaxies is presented. The detailed central morphology and structural properties of these objects were analyzed by fitting ellipses to isophotes. New central peculiar structures have been identified like, for instance, three double-barred systems (ESO 215-G031; ESO 320-G030; ESO 443-G017), one object with a nested nuclear spiral structure at the center of a primary bar (NGC 5135), one object with a nuclear bar without evidence of a large-scale bar (NGC 4941), and one galaxy with a likely dissolved secondary bar within a primary one (ESO 508-G005). The J − K radial profile proved to be reasonably well linked with the presence of a starburst, but not with the Seyfert activity. For significant starbursts, the central J −K value is 0.3 − 1.5 magnitude larger than the disc one.